Smoke believed to be white phosphorus diffusing in the sky during a strike on a neighborhood of west Mosul on June 2, 2017, during the ongoing offensive by Iraqi forces to retake the city from Islamic State (IS) group fighters. The Human Rights Watch organization on June 14, 2017, urged the US-led coalition fighting the Islamic State group in Syria and Iraq to protect civilians from the effects of white phosphorus.Karim Sahib—AFP/Getty Images

12:13 PM ET

(BEIRUT) — The U.S.-led coalition fighting the Islamic State group in Syria and Iraq is endangering civilians by using artillery-delivered white phosphorous, Human Rights Watch said Wednesday, after reports that such weapons were used in the Syrian city of Raqqa.

HRW said it was not able to independently verify whether the use of the munitions resulted in any civilian casualties. The northern Iraqi city of Mosul and the northern Syrian city of Raqqa, the extremist group's de facto capital, have been under attack by different groups. The U.S. military says it uses white phosphorous in a lawful way.

White phosphorous burns at extremely high temperatures and can be used to illuminate conflict zones or obscure them with smoke. International law prohibits its use in civilian areas because of its indiscriminate effects, from starting fires to causing excruciating burns for bystanders, according to Human Rights Watch.

"No matter how white phosphorus is used, it poses a high risk of horrific and long-lasting harm in crowded cities like Raqqa and Mosul and any other areas with concentrations of civilians," said Steve Goose, arms director at Human Rights Watch. "US-led forces should take all feasible precautions to minimize civilian harm when using white phosphorus in Iraq and Syria."

HRW, citing research and media reports, referred to several incidents in Raqqa and Mosul where artillery-fired white phosphorous was used. The group said the rationale for the use of the weapon is unclear as the U.S.-led coalition doesn't comment on specific incidents.

In a video released through its Aamaq news agency, IS said the U.S.-led coalition used white phosphorous over Raqqa last Thursday at dusk, when Muslims would have been breaking their Ramadan fasts.

The U.S. military refused to comment on specific allegations after last week's attack in Raqqa but said it uses white phosphorous rounds "in accordance with the law of armed conflict... in a way that fully considers the possible incidental effects on civilians and civilian structures."

HRW said U.S.-led forces in Mosul and Raqqa are using U.S.-made M825-series 155mm artillery projectiles containing 116 felt wedges impregnated with white phosphorus, which ignites and continues to burn when exposed to the air. This is the only type of 155mm white phosphorus projectile in U.S. stocks that can be air-burst, HRW said.

Neither IS nor Syrian government forces are known to possess or to have used the U.S.-made munitions, it said.